Slack-moji support

[notwillk]

Add support for defining the set of custom emojis in slack. Having a unified source would eliminate name permission issues around individuals adding their own while unlocking the advantages of a declarative configuration that terraform provides.

Describe alternatives you've considered In the past I've written one-off scripts to do this, but implementing it via a terraform provider allows a better experience with less code.

Additional context

• Emoji API methods
• Manual process for adding custom emojis

[notwillk]

I'm happy to help, but it's been years since I've written go (and my backlog of side projects is huge), so I won't be quick about it.